1. Field of the Invention
The present invention relates to a thickness extensional piezoelectric resonator using harmonics of a thickness extensional vibration mode, and more particularly, to a thickness extensional piezoelectric resonator for use as a series-arm resonator and a parallel-arm resonator in a ladder-type filter, and a ladder-type filter and piezoelectric resonator component including such a piezoelectric resonator.
2. Description of the Related Art
Piezoelectric resonators are used in various piezoelectric resonator components such as piezoelectric oscillators, discriminators, and piezoelectric filters. Known piezoelectric resonators of this kind utilize various piezoelectric vibration modes, depending on the frequency to be used.
An energy-trap piezoelectric resonator utilizing a higher-mode of a thickness extensional vibration mode is disclosed in Japanese Examined Patent Application Publication No. 63-40491.
That is, a thickness extensional piezoelectric resonator having a plurality of internal electrodes arranged one on top of another and separated by piezoelectric layers made of a piezoelectric ceramic material and where the piezoelectric layers between internal electrodes are polarized in opposite directions alternately in the direction of thickness is known.
However, although the above-mentioned prior art utilizes harmonics of a thickness extensional vibration mode by disposing a plurality of internal electrodes in a piezoelectric body, only a few examples are disclosed regarding the question of where the internal electrodes are located and arranged. That is, in the above-mentioned prior art, only the embodiments where adjacent internal electrodes are spaced 73 xcexcm away from each other and the whole thickness of the piezoelectric body is 259 xcexcm or 257 xcexcm are shown as a piezoelectric resonator utilizing a third-order thickness extensional vibration mode.
The thickness extensional piezoelectric resonators utilizing harmonics of a thickness extensional vibration mode as described above are manufactured using a mother piezoelectric substrate. Then, since warping or other deformities occur in the mother piezoelectric substrate, surface grinding is often performed. Because of the grinding, the location of the internal electrodes varies in the direction of thickness in the piezoelectric substrate and the frequencies therefore fluctuate. In particular, the higher frequencies in which the thickness extensional piezoelectric resonator is used, the more frequency fluctuations that the piezoelectric resonator exhibits, and this fact is an obstacle to using a thickness extensional piezoelectric resonator for higher frequencies.
Up to now, a ladder-type filter having a series-arm resonator and a parallel-arm resonator is widely used as a bandpass filer. In the ladder-type filter in which the series-arm resonator and parallel-arm resonator are composed of energy-trap thickness extensional piezoelectric resonators, the package construction can be simplified. However, when the above-mentioned piezoelectric resonator utilizing harmonics of a thickness extensional vibration mode is used, because the accuracy of resonant frequencies and antiresonant frequencies are not adequate as described above, it is difficult to achieve excellent filtering characteristics.
In order to overcome the problems described above, preferred embodiments of the present invention provide an energy-trap thickness extensional piezoelectric resonator utilizing harmonics of a thickness extensional vibration mode having a greatly increased accuracy of frequencies and operating at much higher frequencies, a thickness extensional piezoelectric resonator defining a series-arm resonator and a parallel-arm resonator of a ladder-type filter, and a piezoelectric resonator component.
Other preferred embodiments of the present invention provide a ladder-type filter including a thickness extensional piezoelectric resonator according to other preferred embodiments of the present invention, which defines a series-arm resonator or a parallel-arm resonator, such that excellent filtering characteristics are achieved and higher frequencies can be used.
A first preferred embodiment of the present invention provides an energy-trap thickness extensional vibration mode piezoelectric resonator to be used as a series-arm resonator of a ladder-type filter including a piezoelectric body uniformly polarized in the direction of thickness and N number of internal electrodes, where N is an integer equal to 3 to 5, disposed in the piezoelectric body and stacked on each other with piezoelectric layers disposed therebetween. The (Nxe2x88x921)th higher-order mode of a thickness extensional vibration mode generated by applying electric fields of opposite polarity alternately in the direction of thickness to piezoelectric layers between internal electrodes is utilized. When the thickness of a piezoelectric layer between adjacent internal electrodes in the direction of thickness is denoted by D and the thicknesses of a first and second piezoelectric layer outside the outermost internal electrodes in the direction of thickness are denoted by D1 and D2, the following relationships are defined: 0.50xe2x89xa6(D1+D2)/2Dxe2x89xa61.00 at N=3, 0.50xe2x89xa6(D1+D2)/2Dxe2x89xa60.90 at N=4, and 0.50xe2x89xa6(D1+D2)/2Dxe2x89xa60.80 at N=5.
A second preferred embodiment of the present invention provides an energy-trap thickness extensional vibration mode piezoelectric resonator defining a parallel-arm resonator of a ladder-type filter including a piezoelectric body uniformly polarized in the direction of thickness and N number of internal electrodes, where N is an integer equal to 3 to 5, which are disposed in the piezoelectric body and stacked on each other with piezoelectric layers disposed therebetween. The (Nxe2x88x921)th higher-order mode of a thickness extensional vibration mode generated by applying electric fields of opposite polarity alternately in the direction of thickness to piezoelectric layers between internal electrodes is utilized. When the thickness of a piezoelectric layer between adjacent internal electrodes in the direction of thickness is denoted by D and the thicknesses of a first and second piezoelectric layer outside the outermost internal electrodes in the direction of thickness are denoted by D1 and D2, the following relationships are satisfied: 0.10xe2x89xa6(D1+D2)/2Dxe2x89xa60.80 at N=3, 0.10xe2x89xa6(D1+D2)/2Dxe2x89xa60.50 at N=4, and 0.10xe2x89xa6(D1+D2)/2Dxe2x89xa60.45 at N=5.
A third preferred embodiment of the present invention provides an energy-trap thickness extensional vibration mode piezoelectric resonator defining a series-arm resonator of a ladder-type filter including a piezoelectric body and N number of internal electrodes, wherein N is equal to an integer equal to 3 to 5, disposed in the piezoelectric body and stacked on each other with piezoelectric layers disposed therebetween. The piezoelectric layers between internal electrodes are polarized in opposite direction alternately in the direction of thickness and the (Nxe2x88x921)th higher-order mode of a thickness extensional vibration mode is utilized. When the thickness of a piezoelectric layer between adjacent internal electrodes in the direction of thickness is denoted by D and the thicknesses of a first and second piezoelectric layer outside the outermost internal electrodes in the direction of thickness are denoted by D1 and D2, the following relationships are satisfied: 0.60xe2x89xa6(D1+D2)/2Dxe2x89xa61.10 at N=3, 0.65xe2x89xa6(D1+D2)/2Dxe2x89xa60.90 at N=4, and 0.60xe2x89xa6(D1+D2)/2Dxe2x89xa60.80 at N=5.
A preferred embodiment of the present invention provides an energy-trap thickness extensional vibration mode piezoelectric resonator defining a parallel-arm resonator of a ladder-type filter including a piezoelectric body and N number of internal electrodes, where N is an integer equal to 3 to 5, disposed in the piezoelectric body and stacked on each other with piezoelectric layers disposed therebetween. The piezoelectric layers between internal electrodes are polarized in opposite direction alternately in the direction of thickness and the (Nxe2x88x921)th higher-order mode of a thickness extensional vibration mode is utilized. When the thickness of a piezoelectric layer between adjacent internal electrodes in the direction of thickness is denoted by D and the thicknesses of a first and second piezoelectric layer outside the outermost internal electrodes in the direction of thickness are denoted by D1 and D2, the following relationships are satisfied: 0.10xe2x89xa6(D1+D2)/2Dxe2x89xa61.10 at N=3, 0.1023 (D1+D2)/2Dxe2x89xa60.90 at N=4, and 0.10xe2x89xa6(D1+D2)/2Dxe2x89xa60.80 at N=5.
A ladder-type filter according to one of the preferred embodiments of present invention includes a series-arm resonator and a parallel-arm resonator. The series-arm resonator includes a thickness extensional vibration mode piezoelectric resonator according to the first or third preferred embodiment of the present invention.
A ladder-type filter according to another preferred embodiment of the present invention includes a series-arm resonator and a parallel-arm resonator. The parallel-arm resonator includes a thickness extensional vibration mode piezoelectric resonator according to the second or fourth preferred embodiment of the present invention.
In a thickness extensional vibration mode piezoelectric resonator according to the first through fourth preferred embodiments of the invention, the plurality of internal electrodes preferably include linear electrodes and intersect with each other through piezoelectric layers, and the intersection portion constitutes an energy-trap type piezoelectric vibration portion.
A piezoelectric resonator component according to a further preferred embodiment of the present invention includes at least a thickness extensional vibration mode piezoelectric resonator according to any one of the first to fourth preferred embodiments of the present invention, a case substrate bonded to the piezoelectric resonator with a space for allowing the piezoelectric resonator to vibrate unhindered, and a conductive cap bonded to the case substrate so as to enclose the piezoelectric resonator.
Other features, elements, characteristics and advantages of the present invention will become more apparent from detailed description of preferred embodiments thereof with reference to the attached drawings.